Making a threading dial

[stevec]

(09-01-2012, 10:44 PM)Mayhem Wrote: I had originally been told that as my leadscrew is 2tpi,

Mayhem, I'm certainly not "calling" you on this but I'm having trouble picturing a leadscrew of 2 tpi. The leadscrew on my 16 x 60 is 1.5" in Dia. and it's pitch is 4 TPI and I'm trying to picture the helix of 2 tpi.
What's the Dia. of your leadscrew?

[Mayhem]

(09-02-2012, 07:16 AM)stevec Wrote: Mayhem, I'm certainly not "calling" you on this but I'm having trouble picturing a leadscrew of 2 tpi. The leadscrew on my 16 x 60 is 1.5" in Dia. and it's pitch is 4 TPI and I'm trying to picture the helix of 2 tpi.
What's the Dia. of your leadscrew?

I'll have to go and measure it Steve but here is a photo of one of the half nuts from when I stripped her down:

   

Here is a pic showing the leadscrew when I had it chucked up to clean it but it isn't the best.

   

[Highpower]

(09-02-2012, 05:35 AM)Hopefuldave Wrote: Often the dial's attached to the gear's shaft with a friction fit, so for long, fine, odd-or-half-tpi threads you can rotate it to bring a number up approaching the index mark before you start threading - this can save a lot of time!

Dave H. (the other one)

Dave brings up a good point here. At one time I started having a problem with not being able to engage the half-nuts on my lathe when the dial marks came around. They would only fall in ahead of, or behind the dial markings.

Then I discovered the bolt that holds the drive gear on the dial shaft had loosened up and allowed the gear to turn on the shaft a bit. The gear is not keyed to the shaft.

Engaging the half-nuts on the feed screw (lathe off) I turned the dial by hand until one of the marks lined up, and then re-tightened the bolt holding the gear in place. Problem solved.

[Hopefuldave]

Looking at the pics, it's pretty definitely 2 tpi... A lot of older lathes had relatively coarse leadscrews as most of the threads they had to cut were pretty coarse too :)

If you're making a nice new numbered dial with a friction fit to replace the old centre-punched one, maybe it'll be worth knurling the edge for adjustments?

================== Probably unwanted info starts here ==========

If you're making a new gear too it's probably going to want to be 16 teeth, as an 8-tooth gear will have some funny tooth shapes and quite a pronounced helix... even a straight-cut 16-tooth gear gets tricky with a 14.5-degree pressure angle!

Some calculations for the gear (if you need to):

Assuming you have a 29-degree Acme leadscrew so want 14/5-degree pressure-angle gears - bear in mind this is a "rough" gear, you'd have some errors creeping in from setting it over to match the helix angle of the leadscrew... Using something soft like brass, aluminium, delrin or nylon would be a good idea, the gear would wear instead of the leadscrew...

The gear won't be a particular whole-number diametral pitch (the pitch will be 1/2" *circular*), you could calculate it as Pi/(a half) or 2 pi DP - not an off-the-shelf gear!

the initial diameter of the gear blank would be (teeth + 2)/2 pi - so for a 16-tooth, 18/(2 pi), OD = 2.865" - you could reduce this a touch for clearance without much of a problem.

If your leadscrew has a keyway and the gear's made of something *soft*, you could then "gash" the gear blank for the 16 tooth spaces and slowly feed it (rotating on a bearing) in against the rotating leadscrew, it'd cut its own tooth profile?
If not, a hob copying the leadscrew (diameter, pitch and thread form) and fluted like a tap will do a very good job if run in the lathe with the (gashed) gear blank free-running against it.

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Interesting piece of vintage iron, that - the back-gear cover looks very familiar, but I can't place it...

Dave H. (the other one)

[ScrapMetal]

Someone double-check me on this...

Looking at the pic with the tape measure starting at the "60" mark and reading to the left I'm reading 23mm for two "turns". When I do a conversion 1" = 25.4mm, so if that is supposed to be a 2 tpi thread something would seem to be off or are we dealing with a metric thread?

Thanks,

-Ron

[Mayhem]

I had to stop by the workshop today, so I snapped a couple of quick pics. The more detailed stuff will have to wait until the weekend.

I measured the leadscrew diameter and it came out at 44.03mm which is basically 1-3/4" (it was hard to keep the mic perpendicular and straddling the major diameter). It is definitely 2tpi when measured with an inch rule and also an ACME thread gauge:

   
   

I also took a couple of pics of the thread dial:

   
   
   

[TOM REED]

Interesting old lathe, that gear on the thread dial sure looks to be thin especially compared to my lathe. tom

[SnailPowered]

It probably just looks thin in comparison to the rest of the massively oversized parts I was thinking the same thing though.

[dallen]

might be a replacement someone filed out of thin brass stock.

[stevec]

Is the apparent tilt of the threading dial axis actually there or is it some kinda distortion of the pic.
If so it appears to me to be tilted in the wrong direction. Both dials on my machines axis are 90° to the leadscrew.
I agree with dallen's thought, that the gear may have been cobbled up by a previous owner. Maybe long after cleaning out the chip pan the operator/owner noticed the original gear had fallen off.